The present invention relates to filtering signals which are received by a television receiver. Specifically a bandpass filter is provided for filtering the signal which is applied to a television receiver antenna terminal.
Recently a number of accessory devices have been available for generating signals containing information for displaying images or symbols on a television receiver. These devices include video games and video recorders which when operatively connected to the antenna terminals of a television receiver cause images to be displayed on the television receiver picture tube.
These devices all generate a radio frequency (RF) carrier signal having a frequency identical to that of a television station broadcast carrier. The carrier is modulated with picture and/or sound information and a composite signal centered at the frequency of a television broadcast channel is generated. The composite signal is applied to the antenna terminals of the television receiver. The television receiver when tuned to a channel corresponding to the channel frequency of the transmitted composite signal processes the composite signal producing an image and/or sound.
The information bearing radio frequency signal provided by these devices must have a frequency spectrum substantially the same as the signal provided by a broadcast television transmitter. In the NTSC system this modulated signal has a bandwidth of substantially 6 mHz.
The bandwidth limitations on a signal produced by an accessory device are imposed by a bandpass filter having a bandwidth of 6 mHz. The filter has a passband coinciding with the frequency spectrum occupied by a television broadcast signal. In order to preserve the information contained in the signal, the filter must be phase linear over the 6 mHz bandwidth.
Some of the devices which generate signals for application to a television receiver antenna terminal do so on more than one carrier frequency. Therefore it is desirable to provide a bandpass filter having a passband which may be centered at one of two television channels. The bandwidth of the filter must remain 6 mHz for each channel which is filtered, and the phase linearity of the filter over the 6 mHz bandwidth must also remain constant.